The invention relates to a device for thermally killing weeds near plants of a valuable crop arranged in rows, comprising a frame which is intended to be moved in the direction of the rows of plants, one or more burners fixed to the frame, the nozzles of which are directed at or just above the ground, discharge means for combustion gases fitted at some distance opposite the burner nozzles, and feed means for feeding in cooling medium to form a cooling curtain at some distance above the place where the combustion gases of the burners will touch the ground.
Such a device is known from U.S. Pat. No. 3,477,174.
Chemical herbicides are commonly used for killing weeds. They usually constitute a serious assault on the environment. Pollution of the soil and pollution of ground and surface water have become an increasing threat in recent years. Thermal killing of weeds has become known as an ecologically excellent alternative to chemicals. For some time now the Dutch firm HOAF Apparatenfabriek has been putting equipment on the market by means of which undesired vegetation is exposed for a short period to such a high temperature that the leaves and stems die off and seeds lying on the surface of the ground become infertile. Since the duration of heating is short, micro-organisms in the ground are not adversely affected. The burning of propane or another gaseous or liquid hydrocarbon produces infrared radiation which is aimed directly at the weeds. A disadvantage of the known method is that it is not suitable for killing weeds around the bottom part of the stems of cultivated crops such as young maize plants. Although the bottom part of the stems of those crops can itself withstand the very temporary heat, the rising heat still damages to top parts of the stems and the leaves of the plants. This problem is partially solved by the device according to U.S. Pat. No. 3,477,174. This device has a number of deflector plates which are situated close to the ground and are placed next to each other and spaced apart. The burners are placed in front of these deflector plates and the heat developed by the burners is concentrated below the deflector plates for thermally killing the weeds between the rows of plants. The gases flowing inwards and upwards through the openings between the deflector plates are concentrated around the base of the plants by the cooling curtain extending between and above the deflector plates. The combustion gases flow in one direction essentially parallel to the row of plants and at right angles to the cooling air stream coming out of air nozzles placed opposite each other. When the device is passing, the plant parts above the base of the plants are in a chamber in which both the hot combustion gases and the cooling air are flowing. The main disadvantage of this device is that the gas consumption will be high in order to kill both the weeds growing directly around the plant stems and the weeds between the rows of plants. The weeks between the rows of plants could be removed simply and cheaply by mechanical hoeing. Another disadvantage is that the hot combustion gases, although cooled by the air streams, move up along the plant stems in the above-mentioned chamber, so that the plant parts situated above the plant stems will still be heated.